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<html lang="en">

  <head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>案例四</title>
  </head>

  <body>

  </body>
  <script>
    // 案例3
    var a = 18;
    f1();

    function f1() {
      var b = 9;
      console.log(a);
      console.log(b);
      var a = '123';
    }

    //解析过程
    /*  var a;

     function f1() {
       var b;
       var a;
       b = 9;
       console.log(a); //undefined
       console.log(b); //9
       a = '123';
     }
     a = 18;
     f1(); */
  </script>

</html>